Mobile IP is proposed as a method of, when a mobile terminal having an IP (internet protocol) address moves from a subnet that partially constitutes multiple IP networks to another subnet that partially constitutes the multiple IP networks, making it possible for the mobile terminal to maintain communications without interrupting sessions. The mobile IP is defined by RFC (Request for Comment: Document including information about the Internet) 2002 of IETF (Internet Engineering Task Force: Internet technology special investigation committee).
An example of a mobile network using the mobile IP is shown in FIG. 17. In the figure, reference numerals 1a and 1b denote IP networks, reference numeral 2 denotes a network gateway, reference numeral 3 denotes a mobile terminal, reference numerals 4a, 4b, and 4c denote base stations, reference numeral 5 denotes a foreign agent (FA), and reference numeral 7 denotes a communication node (CN).
The IP networks 1a and 1b are different IP networks that constitute a set of multiple IP networks, and operate according to different protocols. Each of the IP networks 1a and 1b has a plurality of routers. It is assumed that the IP network 1b shown in the figure is the Internet. The IP networks 1a and 1b are connected to each other by way of the network gateway 2 that makes these IP networks interwork.
The mobile terminal 3 shown in the figure joins the IP network 1a, and obtains service provided by the IP network 1a under normal circumstances. In other words, the IP network 1a is a home network, i.e., a home domain for the mobile terminal 3 shown in the figure. The mobile terminal 3 has a function of communicating with another mobile terminal according to the mobile IP, and can obtain service even if it moves to another IP network included in the multiple IP networks.
The FA 5 belongs to another subnet that partially constitutes the multiple IP networks, and provides an address (i.e., a care-of address) specifying the other subnet, to which the FA 5 belongs, to mobile terminals that have moved to the other subnet from still other subnets. The base stations 4a and 4b shown in the figure belong to the same subnet as that to which the FA 5 belongs. Although only the FA 5 is shown in FIG. 17, two or more subnets can be connected to the IP network 1a in the multiple IP networks.
According to the mobile IP, the HA 6 is placed within the home domain (e.g., the IP network 1a) of a mobile terminal (e.g., the mobile terminal 3). An IP address (i.e., a home address) that can be used in this home domain is assigned to any mobile terminal that is under the charge of the IP network 1a. The mobile terminal 3 has a storage unit (not shown in the figure) for storing the home address thereof, and the HA 6 has a storage unit (not shown in the figure) for storing the home addresses of all mobile terminals that join the subnet thereof. When the mobile terminal has moved to a location where it can obtain service provided by another subnet, the HA 6 manages the address of another FA 5 that belongs to the other subnet where the mobile terminal 3 exists now as the location of the mobile terminal 3. Therefore, when the mobile terminal 3 detects that it has moved from an area that is under the charge of an old FA or the HA 6 to another area that is under the charge of a new FA 5, the mobile terminal 3 registers an address (referred to as a care-of address) specifying the new FA 5 as the address indicating the destination of the mobile terminal 3 in the HA 6 by way of the new FA 5.
When the CN 7 linked with the IP network 1b transmits an IP packet to the mobile terminal 3, the HA 6 that manages the care-of address of the mobile terminal 3 catches the IP packet, which are destined for the home address of the mobile terminal 3. Then, the HA 6 transfers the caught IP packet to a registered FA (in the case of FIG. 17, the FA 5) by way of an IP tunnel. Here, transfer of IP packets via an IP tunnel means transfer of the IP packets to an FA with encapsulation of IP packets that the HA has received into IP packets in another format having an IP header indicating a sending source address (e.g., the IP address specifying the HA 6) and a destination address (e.g., the care-of address specifying the FA 5). When receiving an encapsulated IP packet, the FA retrieves an original IP packet from the encapsulated IP packet and then transfers it to the mobile terminal 3 that is the destination. This mechanism implements the mobile transmissivity of the mobile terminal 3 in accordance with the mobile IP.
On the other hand, as a method of implementing IP packet transmission from the mobile terminal 3 to the CN 7, there have been provided a method of transmitting IP packets by way of a normal routing route and a method of transmitting IP packets by way of a tunnel between an FA and an HA (the latter method is called a reverse tunnel method).
A problem encountered with a prior art mobile network constructed as mentioned above is that loads are concentrated on an HA, as explained below.
Because the HA 6 has a function of performing signaling processing according to the mobile IP (e.g., calling for the destination) and a function of performing IP tunnel transfer processing on user packets, the traffic and load are concentrated on the HA. In addition, because while a plurality of FAs can be installed at geographically dispersed areas to which the mobile terminal 3 can move, only one HA is installed in each home domain (i.e., each subnet), the load is further concentrated on the HA. Therefore, the HA easily becomes a bottleneck on the network, and the trouble tolerance of the network is reduced. A further problem arises in the scalability of the multiple IP networks because the load on each HA increases with an increase in the number of subnets.
The problem of such concentration of loads can arise in the FA 5 that makes a report about the roaming of a mobile terminal to the HA 6, and that decapsulates packets to be transmitted to the destination and transfers them to the destination.
Furthermore, triangular routing as shown in FIG. 17 can be provided because IP packets destined for a mobile terminal are basically transmitted to a corresponding FA by way of the HA. More concretely, IP packets to be transmitted from a mobile terminal to the CN 7 are transmitted from the FA 5 to the network gateway 2 by way of one or more routers without using a reverse tunnel passing through the HA. Therefore, there causes a time delay different between the forward and backward directions of transmission because IP packets are passed through a route different depending on the direction of transmission of the IP packets, and this can result in the occurrence of trouble.